This invention relates to an image forming apparatus utilizing an electrophotographic method such as, for example, a copying machine, a printer, and a facsimile machine.
At present, as regards an image forming apparatus of a certain kind utilizing an electrophotographic method, it is widely utilized an image forming apparatus of a reverse development type in which the surface of a drum-shaped photoreceptor which is an image forming member and is driven to rotate is charged to the specified polarity (negative, for example) by a charging means consisting of a charging device using, for example, corona discharge, an electrostatic latent image is formed by exposing the surface of this photoreceptor image-wise to an image exposure means using, for example, a laser optical system, and the electrostatic latent image is visualized through the toner deposition on the image-wise exposed area of the photoreceptor by a developing means using a two-component developer composed of toner particles which have been triboelectrically charged to the same polarity as the surface potential of the photoreceptor (negative, for example) and carrier particles (positively charged). In this case, in making toner particles adhere to the image-wise exposed area of the photoreceptor, a development bias voltage of the same polarity as the surface potential of the photoreceptor (negative polarity, for example) is applied to a developing roller (developer carrying member) which makes up a developing means and is arranged opposite to the photoreceptor with the developing region positioned in between.
When an image formation sequence is started by means of such an image forming apparatus, problems of various kinds are produced in some cases, depending on the relationship between the start timing of the application of the charging voltage and the start timing of the application of the development bias voltage. In the following, such problems will be explained with reference to FIG. 5 and FIG. 6.
For example, as shown in FIG. 5, for the non-print area of the photoreceptor from which no toner image is to be transferred to an image recording medium such as a transfer paper sheet (hereinafter referred to as a recording medium simply) which is positioned, with respect to the rotating direction of the photoreceptor, at the downstream side of the print area of the photoreceptor from which a toner image is to be transferred to the recording medium, in the case where a condition is set in such a way that the development bias application region (Ra) to which a development bias voltage is to be applied substantially coincides with the region where the surface potential of the charged photoreceptor is raised to a specified potential value (Rb), to state it concretely, in the case where a condition is set so as to make the start point on the photoreceptor of the application of the charging voltage (Pb1) come to a more downstream position than the start point on the photoreceptor of the application of the developing bias voltage (Pa1), a xe2x80x9ccarrier adheringxe2x80x9d phenomenon, in which carrier particles in the developer carried on the developing sleeve adheres on the surface of the photoreceptor, is produced in the region between a place located in the charging voltage application region facing the charging means at the starting time of the application of the charging voltage (Pb1) (for example, the place which is located at the downmost stream position with respect to the rotating direction of the photoreceptor in the charging voltage application region (i), refer to FIG. 1), and the place where the surface potential has the same potential value as the development bias voltage (Vd) at the rising time of the surface potential (the place which is located in the developing region P at the starting time of the application of the development bias voltage (Pa1) (ii)). In this case, compared to the rise characteristic at the time of application of the development bias voltage shown by the wave form (A) in FIG. 5 being sharp, the rise characteristic of the charge potential of the photoreceptor at the time of the application of the charging voltage is more gentle; therefore, by the setting of a condition to make the start point of the application of the charging voltage (Pb1) more downstream than the start point of the application of the development bias voltage (Pa1), it is obtained a state in which the development bias application region (Ra), which means the region being subjected to the application of the development bias voltage, comes to substantially coincide with the region where the charge potential has the specified potential value (Rb).
Further, also at the ending time of an image formation sequence, in the case where the stop point on the photoreceptor of the application of the charging voltage (Pb2) is determined to be more upstream than the stop point on the photoreceptor of the application of the development bias voltage (Pa2), a xe2x80x9ccarrier adheringxe2x80x9d phenomenon, in which carrier particles in the developer carried on the developing sleeve adhere on the photoreceptor surface, is produced in the region between the place where the surface potential of the charged photoreceptor has the same potential value as the development bias voltage (Vd), which is lower than the specified surface potential (Vh) at the falling time of the surface potential (the place which is located at the developing region P at the stopping time of the development bias voltage (Pa2)), and the place which is located at the developing region P at the stopping time of the application of the charging voltage (Pb2).
On the other hand, for example, as shown in FIG. 6, in the case where, in order that the development bias voltage may be applied even to the region where the surface potential of the charged photoreceptor has not been raised to the specified potential value, when an image formation sequence is started, the application region of the development bias voltage (Ra) is determined to be larger than the region (Rb) where the surface potential of the charged photoreceptor is kept at the specified potential state, to state it concretely, in the case where the start point on the photoreceptor of the application of the charging voltage (Pb1) is determined to substantially coincide with the start point on the photoreceptor of the application of the development bias voltage (Pa1), toner particles in the developer carried on the developing sleeve adhere on the surface of the photoreceptor to form, for example, an unnecessary stripe-shaped toner image on the photoreceptor, in the region where the potential difference between the surface potential and the development bias voltage is large between a place which is located in the charging voltage application region at the starting time of the application of the charging voltage (Pb1) (for example, the place which is located at the downmost stream position with respect to the rotating direction of the photoreceptor in the charging voltage application region (i)), and the place where the surface potential of the charged photoreceptor has been raised to the same value as the development bias voltage (Vd) at its rising time (ii).
Further, also at the ending time of an image formation sequence, in the case where the stop point on the photoreceptor of the application of the charging voltage (Pa2) is determined to substantially coincide with the stop point on the photoreceptor of the development bias voltage (Pb2), toner particles in the developer carried on the developing sleeve adhere on the surface of the photoreceptor to form, for example, an unnecessary stripe-shaped toner image on the photoreceptor in the region where the difference between the surface potential and the development bias voltage is large between the place where the surface potential of the charged photoreceptor has the same potential value as the development bias voltage (Vd), which is lower than the specified potential value (Vh), at its falling time, and the place which is located at the developing region P at the stopping time of the application of the development bias voltage (Pa2).
In the above, the reason that the rise characteristic and the fall characteristic of the surface potential of the charged photoreceptor at the starting time and the stopping time of the charging voltage are gentle (gently-sloping) is that, in contrast with the development bias voltage acting in the narrow developing region (development nip), the charging voltage acts on the photoreceptor with a broad width of a certain degree as is observed in the example of a corona discharge electrode; hence, the rise characteristic or the fall characteristic of the surface potential on the charged photoreceptor becomes more gentle than the rise characteristic or the fall characteristic of the charging power source itself.
Therefore, in a conventional image forming apparatus, for the reason that a damage tends to be produced on the surface of the photoreceptor if carrier adhering is produced, which lowers the characteristics of the photoreceptor itself, it is made a setting such that image forming operation is practiced under an operational condition to positively form an useless toner image (refer to FIG. 6), and it is put into practice to remove the toner image which is inevitably formed on the photoreceptor in order to prevent the occurrence of carrier adhering by an image forming member cleaning means. For the image forming member cleaning means, it can be cited a blade cleaning method in which toner particles remaining on the photoreceptor are removed by, for example, a cleaning blade being provided in a state of pressing contact with the surface of the photoreceptor and rubbing the surface of the photoreceptor.
However, in the case where an image formation operation is carried out in a state controlled in this way, the cleaning member in the image forming member cleaning means comes to have an excessive load, and as the result of it, it is raised a problem that it becomes impossible to remove toner particles remaining on the photoreceptor with certainty which makes it difficult to form a high-quality image after all.
On the other hand, in recent years, as regards an image forming apparatus as described in the above, a requirement for making it of a high image quality is strong, and in order to form an image having the same image quality as that of an image formed by offset printing, for example, it has been put into practice to make toner particles composing a toner have a small diameter (for example, refer to the publications of the unexamined patent application 2000-81722 and 2001-5208). Especially, by the use of a toner which is composed of toner particles having small particle diameter and has a sharp particle size distribution, the packing density of the toner particles in the toner layer is made higher and the vacant space is reduced, which gives a high-quality image.
However, accompanied by the toner particles being made to have a smaller particle diameter, the adhering force between the toner particles and the photoreceptor becomes larger; as the result, it becomes extremely difficult to remove untransferred toner particles including the residual toner particles after transfer on the photoreceptor, and the occurrence of a poor-cleaning phenomenon of what is called xe2x80x9cslipping-throughxe2x80x9d (also referred to as xe2x80x9cinsufficient toner removalxe2x80x9d) becomes remarkable.
Especially, in the case where an image formation operation is carried out in an environment of a low temperature and a low humidity, a phenomenon of unsatisfactory cleaning such as the slipping-through of toner particles appears remarkably.
A phenomenon as described in the above is produced not only at the starting time of an image formation sequence but also at the ending time of an image formation sequence.
This invention has been made on the basis of the above-mentioned situation, and its object is to provide an image forming apparatus which is capable of exhibiting a high cleaning effect with certainty by a cleaning means over a long period of time, and hence, is capable of forming an image of a high image quality.
The object of this invention can be accomplished by the adopting of any one of the structures (1) to (17) shown below.
(1) An image forming apparatus comprising an image forming member to be rotated, a charging means for charging the surface of the image forming member, an exposure means for exposing the surface of the image forming member to light to form an electrostatic latent image, a developing means for reverse-developing the electrostatic latent image through conveying a layer of two-component developer containing toner particles having a volume-average particle diameter of 3 xcexcm to 5 xcexcm by a developer carrying member provided rotatably opposite to the image forming member with the developing region positioned in between with a development bias applied, a transfer means for transferring a toner image formed on the image forming member onto a recording medium or onto an intermediate transfer member in a transfer region, and a cleaning means for removing untransferred toner particles remaining on the image forming member having passed the transfer region by a cleaning member provided in pressing contact with the surface of the image forming member, characterized in that, with the length in the lengthwise direction of an useless toner image formed on the image forming member at the starting time and the ending time of an image formation sequence denoted by L, the total charge quantity of the toner particles making up the useless toner image is 0.04L xcexcC or less.
(2) An image forming apparatus comprising an image forming member to be rotated, a charging means for charging the surface of the image forming member, an exposure means for exposing the surface of the image forming member to light to form an electrostatic latent image, a developing means for reverse-developing the electrostatic latent image through conveying a layer of two-component developer containing toner particles having a volume-average particle diameter of 3 xcexcm to 5 xcexcm by a developer carrying member provided rotatably opposite to the image forming member with the developing region positioned in between with a development bias applied, a transfer means for transferring a toner image formed on the image forming member onto a recording medium or onto an intermediate transfer member in a transfer region, and an image forming member cleaning means for removing untransferred toner particles remaining on the image forming member having passed the transfer region by a cleaning member provided in pressing contact with the surface of the image forming member, characterized in that, at the starting time of the image formation sequence, a specified exposure processing is carried out in which exposure by the exposure means is started at the timing when the place on the image forming member located at the downmost stream position with respect to the rotating direction of the image forming member in the charging voltage application region on the image forming member facing the charging means at the starting time of the application of the charging voltage by the charging means reaches the exposure position by the exposure means, or before the place reaches the exposure position, and exposure by the exposure means is stopped at the timing when the place on the image forming member located at the uppermost stream position in the charging voltage application region at the starting time of the charging voltage application reaches the exposure position or after the place has passed the exposure position, further, a development bias voltage is applied to the developer carrying member at the timing when the place on the photoreceptor located at the exposure position at the stopping time of the exposure in the specified exposure processing reaches the developing region or before the place reaches the developing region, and with the length in the lengthwise direction of an useless toner image formed on the image forming member denoted by L, the total charge quantity of the toner particles making up the useless toner image is 0.04L xcexcC or less.
(3) An image forming apparatus as set forth in the structure (2) characterized in that the start of the application of the development bias voltage to the developer carrying member is carried out during a period of time before the timing when the place on the image forming member which is located at the exposure position at the exposure stopping time in the specified exposure processing reaches the developing region and after the timing when the place located at the 15 mm downstream position of the place with respect to the rotating direction of the image forming member reaches the developing region.
(4) An image forming apparatus as set forth in the structure (2) or (3) characterized by the total quantity of the toner particles forming the stripe-shaped useless toner image formed on the image forming member being made 25 mg or less.
(5) An image forming apparatus as set forth in any one of the structures (2) to (4) characterized in that the stopping of the exposure in the specified exposure processing is carried out, between the timing when the first place on the image forming member located at the uppermost stream position in the charging voltage application region at the starting time of the charging voltage application reaches the exposure position, and the timing when the second place located, with respect to the moving direction of the image forming member, at the 20 mm or more downstream position of the leading edge position of the beginning image region located at the upstream side of the first place with respect to the rotating direction of the image forming member reaches the exposure position.
(6) An image forming apparatus as set forth in any one of the structures (1) to (5) characterized by the toner particles removed from the image forming member by the image forming member cleaning means being utilized again.
(7) An image forming apparatus comprising an image forming member to be rotated, a charging means for charging the surface of the image forming member, an exposure means for exposing the surface of the image forming member to light to form an electrostatic latent image, a developing means for reverse-developing the electrostatic latent image through conveying a layer of two-component developer containing toner particles having a volume-average particle diameter of 3 xcexcm to 5 xcexcm by a developer carrying member provided rotatably opposite to the image forming member with the developing region positioned in between with a development bias applied, a transfer means for transferring a toner image formed on the image forming member onto a recording medium or onto an intermediate transfer member in a transfer region, and an image forming member cleaning means for removing untransferred toner particles remaining on the image forming member having passed the transfer region by a cleaning member provided in contact with the surface of the image forming member, characterized in that, at the ending time of the image formation sequence, a specified exposure processing is carried out in which exposure by the exposure means is started at the timing when the place located at the downmost stream position with respect to the rotating direction of the image forming member in the charging voltage application region on the image forming member facing the charging means at the stopping time of the application of the charging voltage by the charging means reaches the exposure position by the exposure means, or before the place reaches the exposure position, and exposure by the exposure means is stopped at the timing when the place on the image forming member located at the uppermost stream position in the charging voltage application region at the stopping time of the charging voltage application reaches the exposure position or after the place has passed the exposure position, further, the application of a development bias voltage to the developer carrying member is stopped at the timing when the place on the image forming member located at the exposure position at the starting time of the exposure in the specified exposure processing reaches the developing region or after the place has passed the developing region, and with the length in the lengthwise direction of an useless toner image formed on the image forming member denoted by L, the total charge quantity of the toner particles making up the useless toner image is made 0.04L xcexcC or less.
(8) An image forming apparatus as set forth in the structure (7), characterized in that the stop of the application of the development bias voltage to the developer carrying member is carried out between the timing when the place on the image forming member located at the exposure position at the exposure starting time in the specified exposure processing reaches the developing region and the timing when the point located at the 15 mm upstream position of the place with respect to the rotating direction of the image forming member reaches the developing region.
(9) An image forming apparatus as set forth in the structure (7) or (8), characterized by the total quantity of toner forming the stripe-shaped useless toner image formed on the image forming member being made 25 mg or less.
(10) An image forming apparatus as set forth in any one of the structures (7) to (9), characterized in that the exposure starting in the specified exposure processing is carried out between the timing when the place located, with respect to the moving direction of the image forming member, at the 20 mm or more upstream position of the trailing edge position of the last image area reaches the exposure position and the timing when the place on the image forming member located at the downmost stream position in the charging voltage application region at the stopping time of the charging voltage application reaches the exposure position.
(11) An image forming apparatus as set forth in any one of the structures (7) to (10), characterized by the toner particles removed from on the image forming member by the image forming member cleaning means being utilized again.
(12) An image forming apparatus comprising an image forming member to be rotated, a charging means for charging the surface of the image forming member, an exposure means for exposing the surface of the image forming member to light to form an electrostatic latent image, a developing means for reverse-developing the electrostatic latent image through conveying a layer of two-component developer containing toner particles having a volume-average particle diameter of 3 xcexcm to 5 xcexcm by a developer carrying member provided rotatably opposite to the image forming member with the developing region positioned in between with a development bias applied, a transfer means for transferring a toner image formed on the image forming member onto a recording medium or onto an intermediate transfer member in a transfer region, and an image forming member cleaning means for removing untransferred toner particles remaining on the image forming member having passed the transfer region by a cleaning member provided in pressing contact with the surface of the image forming member, wherein at the starting time and the ending time of the image formation sequence, a development bias voltage is made to act on the deficiently charged areas of the image forming member, while the peripheral speed of the developer carrying member is controlled, characterized in that, at least at one of the starting time and the ending time of the image formation sequence, in the deficiently charged area on the image forming member passing the developing region with a development bias voltage made to act on it, the peripheral speed of the developer carrying member is controlled in such a way that the maximum development toner quantity is reduced from the set maximum development toner quantity to be supplied for the development of the electrostatic latent image to become 0.3 mg/cm2 or less, and with the length in the lengthwise direction of a useless toner image formed in the deficiently charged area on the image forming member denoted by L, the total charge quantity of the toner particles making up the useless toner image is made 0.04L xcexcC or less.
(13) An image forming apparatus comprising a toner image forming unit provided with an image forming member to be rotated, made up of a charging means for charging the image forming member, an exposure means for exposing the image forming member to light to form an electrostatic latent image, and a developing means for reverse-developing the electrostatic latent image to form a toner image through conveying a layer of two-component developer containing toner particles having a volume-average particle diameter of 3 xcexcm to 5 xcexcm by a developer carrying member provided rotatably opposite to the image forming member with the developing region positioned in between with a development bias applied, arranged in the order of operation with respect to the rotating direction of the image forming member in the outer circumferential region of the image forming member, a primary transfer means for forming a primary transfer toner image through transferring the toner image formed on the image forming member onto an intermediate transfer member in a primary transfer region, a secondary transfer means for forming a secondary transfer toner image through transferring the primary transfer toner image formed on the intermediate transfer member onto an image recording medium in a secondary transfer region, an image forming member cleaning means for removing untransferred toner particles remaining on the image forming member having passed the primary transfer region by a cleaning member provided in pressing contact with the surface of the image forming member, and an intermediate transfer member cleaning means for removing untransferred toner particles remaining on the intermediate transfer member having passed the secondary transfer region by a cleaning member provided in pressing contact with the surface of the intermediate transfer member, wherein at the starting time and the ending time of an image formation sequence, a development bias voltage is made to act on the deficiently charged areas on the image forming member, characterized in that at least at one of the starting time and the ending time of an image formation sequence, a part or the whole of the toner particles making up a useless toner image formed in the deficiently charged area on the image forming member passing the developing region with a development bias voltage made to act on it are transferred to the intermediate transfer member, to be removed from on the intermediate transfer member, and with the length in the lengthwise direction of a toner image formed in the deficiently charged area on the image forming member denoted by L, the total charge quantity of the toner particles making up the useless toner image is made 0.04L xcexcC or less.
(14) An image forming apparatus comprising an image forming member to be rotated, a charging means for charging the surface of the image forming member, an exposure means for exposing the surface of the image forming member to light to form an electrostatic latent image, a developing means for reverse-developing the electrostatic latent image to form a toner image through conveying a layer of two-component developer containing toner particles having a volume-average particle diameter of 3 xcexcm to 5 xcexcm by a developer carrying member provided rotatably opposite to the image forming member with the developing region positioned in between with a development bias applied, a transfer means for transferring a toner image formed on the image forming member onto a recording medium or onto an intermediate transfer member in a transfer region, and an image forming member cleaning means for removing untransferred toner particles remaining on the image forming member having passed the transfer region by a cleaning member provided in pressing contact with the surface of the image forming member, wherein at the starting time and the ending time of an image formation sequence, a development bias voltage is made to act on the deficiently charged areas of the image forming member, characterized in that a toner charge quantity reducing means for reducing the charge quantity of undeveloped toner particles on the image forming member by the application of an alternate-current discharging voltage is provided at an upstream position of the cleaning region by the image forming member cleaning means with respect to the rotating direction of the image forming member, at least at one of the starting time and the ending time of an image formation sequence, on the deficiently charged area of the image forming member passing the developing region with a development bias voltage made to act on it, an electric field for charge elimination is made to act, to make the total charge quantity of the toner particles making up a useless toner image formed on the deficiently charged area of the image forming member 0.04L xcexcC or less, where the length in the lengthwise direction of the useless toner image is denoted by L.
(15) An image forming apparatus as set forth in the structure (14) characterized by the toner charge quantity reducing means reducing the absolute value of the average charge quantity of the untransferred toner particles on the image forming member to 35 xcexcC/g or less.
(16) An image forming apparatus comprising an image forming member to be rotated, a charging means for charging the surface of the image forming member, an exposure means for exposing the surface of the image forming member to light to form an electrostatic latent image, a developing means for reverse-developing the electrostatic latent image to form a toner image through conveying a layer of two-component developer containing toner particles having a volume-average particle diameter of 3 xcexcm to 5 xcexcm by a developer carrying member provided rotatably opposite to the image forming member with the developing region positioned in between with a development bias applied, a transfer means for transferring a toner image formed on the image forming member onto a recording medium or onto an intermediate transfer member in a transfer region, and an image forming member cleaning means for removing untransferred toner particles remaining on the image forming member having passed the transfer region by a cleaning member provided in pressing contact with the surface of the image forming member, wherein at the starting time and the ending time of an image formation sequence, a development bias voltage is made to act on the deficiently charged areas of the image forming member, characterized in that a toner charge quantity reducing means for reducing the charge quantity of undeveloped toner particles on the image forming member by the application of a direct-current discharging voltage of the polarity reverse to the charge polarity of the toner is provided, with respect to the rotating direction of the image forming member, at an upstream position of the cleaning region by the image forming member cleaning means, and at least at one of the starting time and the ending time of an image formation sequence, on the deficiently charged area of the image forming member passing the developing region with a development bias voltage made to act on it, an electric field for charge elimination is made to act, to make the total charge quantity of the toner particles making up a useless toner image formed on the deficiently charged area of the image forming member 0.04L xcexcC or less, where the length in the lengthwise direction of the useless toner image is denoted by L.
(17) An image forming apparatus as set forth in the structure (16) characterized by the toner charge quantity reducing means reducing the absolute value of the average charge quantity of the untransferred toner particles on the image forming member to 35 xcexcC/g or less.
By an image forming apparatus having the above-mentioned structure, the load given to the cleaning member in the image forming member cleaning means is reduced by a large margin by making it 0.04L (xcexcC) or less the total charge quantity of the toner particles making up a useless toner image, which is formed inevitably for the image forming member positively for the purpose of preventing the occurrence of carrier adhering to the image forming member at the starting time and the ending time of the image formation sequence; therefore, the expected cleaning effect can be exhibited with certainty and the untransferred toner particles remaining on the image forming member can be removed with certainty; hence, an image having a high image quality can be obtained with certainty.
By the specified exposure processing being carried out when an image formation sequence is started, in which image forming member is exposed to light before the application of the charging voltage, and the exposure by the exposure means is stopped immediately after the starting time of the application of the development bias voltage, compared to the useless toner image which is formed in the case where it is set a state that the start timing of the charging voltage application is made to approximately coincide with the start timing of the development bias voltage, the width of the useless toner image can be made as small as possible; owing to this, it can be actualized with certainty a state that the total charge quantity of the toner particles making up the useless toner image becomes 0.04L (xcexcC) or less.
Further, by the specified exposure processing being carried out when an image formation sequence is finished, in which the image forming member is exposed to light before the application of the development bias voltage is stopped and the exposure by the exposure means is stopped immediately after the charge potential of the image forming member is reduced to zero, compared to the useless toner image which is formed in the case where it is set a state that the start timing of the charging voltage application is made to approximately coincide with the start timing of the development bias voltage, the width of the useless toner image can be made as small as possible; owing to this, it can be actualized with certainty a state that the total charge quantity of the toner particles making up the useless toner image becomes 0.04L (xcexcC) or less.
Further, because the quantity of the toner making up the useless toner image formed on the deficiently charged region of the image forming member can be made to have a small value equal to or less than a definite value by it that, at least at one of the starting time and the ending time of an image formation sequence, when the deficiently charged region on the image forming member passes the developing region, the maximum development toner quantity is reduced by a specified decrement to become the specified quantity from the maximum development toner quantity during the development of the print area, it can be actualized with certainty a state that the total charge quantity of the toner particles making up the useless toner image becomes 0.04L (xcexcC) or less.
Further, because the quantity of the toner making up the useless toner image formed in the deficiently charged region of the image forming member can be made to have a small value equal to or less than a definite value by it that, under the condition that is set in such a manner that, at least at one of the starting time and the ending time of an image formation sequence, a useless toner image is positively formed on the image forming member for the purpose of preventing the occurrence of carrier adhering to the image forming member, a part or the whole of the toner particles making up the useless toner image formed on the deficiently charged area on the image forming member are transferred onto the intermediate transfer member, it can be actualized with certainty a state that the total charge quantity of the toner particles making up the useless toner image becomes 0.04L (xcexcC) or less.
Further, it can be actualized with certainty a state that the total charge quantity of the toner particles making up the useless toner image becomes 0.04L (xcexcC) or less by it that, at least at one of the starting time and the ending time of an image formation sequence, on the deficiently charged area on the image forming member, an electric field for eliminating charge is made to act, by the toner charge quantity reducing means to which an alternate-current discharging voltage or a direct-current discharging voltage of the polarity reverse to the charge polarity of the toner, to reduce the charge quantity of the toner making up the useless toner image.